# std::variant_alternative, std::variant_alternative_t

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< cpp‎ | utility‎ | variant

Defined in header `<variant>` |  |
template <std::size_t I, class T> struct variant_alternative; /* undefined */ | (1) | (since C++17)
template <std::size_t I, class... Types> struct variant_alternative<I, variant<Types...>>; | (2) | (since C++17)
template <std::size_t I, class T> class variant_alternative<I, const T>; | (3) | (since C++17)
template <std::size_t I, class T> class variant_alternative<I, volatile T>; template <std::size_t I, class T> class variant_alternative<I, const volatile T>; | (3) | (since C++17) (deprecated in C++20)

Provides compile-time indexed access to the types of the alternatives of the possibly cv-qualified variant, combining cv-qualifications of the variant (if any) with the cv-qualifications of the alternative.

Formally,

2) meets the TransformationTrait requirements with a member typedef `type` equal to the type of the alternative with index `I`
3) meets the TransformationTrait requirements with a member typedef `type` that names, respectively, std::add_const_t<std::variant_alternative_t<I,T>>, std::add_volatile_t<std::variant_alternative_t<I,T>>, and std::add_cv_t<std::variant_alternative_t<I,T>>

### Member types

Member type | Definition
type | the type of `I`th alternative of the variant, where `I` must be in `[0, sizeof...(Types))`, otherwise the program is ill-formed.

### Helper template alias

template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; |  | (since C++17)

### Example

Run this code

    #include <variant>
    #include <iostream>

    using my_variant = std::variant<int, float>;
    static_assert(std::is_same_v
        <int,   std::variant_alternative_t<0, my_variant>>);
    static_assert(std::is_same_v
        <float, std::variant_alternative_t<1, my_variant>>);
    // cv-qualification on the variant type propagates to the extracted alternative type.
    static_assert(std::is_same_v
        <const int, std::variant_alternative_t<0, const my_variant>>);

    int main()
    {
        std::cout << "All static assertions passed.\n";
    }

Output:

    All static assertions passed.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2974 | C++17 | out-of-bounds index resulted in undefined behavior | made ill-formed

### See also

variant_sizevariant_size_v (C++17) | obtains the size of the `variant`'s list of alternatives at compile time (class template) (variable template)
std::tuple_element<std::tuple> (C++11) | obtains the type of the specified element (class template specialization)

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